The Chemistry, Properties and Tests of Precious Stones

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possessed by precious stones are those of light and its effects, for

one, diaphaneity, as it is called; it may produce single or double refraction, or polarisation; if reflected, it may produce lustre

have, as stated above, single or double

ill be familiar with the refractive property of a light-beam when passing through a medium denser than atmospheric air. It will be quite sufficient to say tha

fraction; thus, light, in passing through their substance, is deviated, part of it going one way, the other portion going in another d

e), when the mark is doubled and two appear. On rotating this rhomb of spar, one of these marks is seen to revolve round the other, which remains stationary, the moving

it the appearance of poles which have different properties, and the polariscope is an instrument in which

ple-fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails at every point to transmit light, which a double-refracting gem allows

stone is one which is quite opaque, no light of any kind passing through its substance; a "diaphanous" stone is one which is altogether transparent; "semi-diaphanous" means one not altogether transparent, and sometimes calle

se stones which owe their beauty or value to Reflect

eties:-(α) adamantine (which some authorities, experts and merchants subdivide as detailed bel

splendent, when objects are reflected perfectly, but of a lower scale of perfection than the true "adamantine" standard, which is absolutely flawless. When still lower, and the reflection, though maybe fairly good, is somewhat "fuzzy," or is confused or out of focus, it is then merely shining; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it cou

name implies, is th

ing the sheen of s

ry in its name; amber and th

quartz, etc.; some experts subdividing this for greater defining accura

e lustre is perfect as in gold; the latter when

, black, or jet-black; whilst those which reflect all the rays of which light is composed, and in the same number of vibrations, appear white. Between these two extremes of none and all we find a wonderful play and variety of colour, as some gems allow the red rays only to pass a

possessed by certain gems and minerals of becoming phosphorescent on be

city. Perhaps two or all of these may be the cause, for electricity is developed in some ge

a second piece of the same quartz in the dark, becomes highly phosphorescent. Certain gems, also, when merely exposed to light-sunlight for preference-then taken into a darkened room, will glow for a short time. The diamond is one of the best examples of th